1. Field of the Invention
The present invention pertains to atomic force microscopy, and specifically, to an apparatus and method for sensing the deflection of a cantilever of an atomic force microscope.
2. Description of the Prior Art
Atomic force microscopy is directed to sensing the forces between a sharp stylus or tip of, for example, a probe and the surface of a sample to be investigated. The interatomic forces between the two cause a displacement of the tip mounted on the end of a cantilever, and this displacement is indicative of surface characteristics, e.g., topography, of the sample.
An atomic force microscope (AFM) is an apparatus capable of observing these surface characteristics of the sample, which can be on the atomic scale. In one mode of AFM operation, the probe is moved close to the sample surface, and a Van der Waals attractive force acts between an atom at the tip of the probe and an atom on the sample surface. If both atoms move close to each other, so as to nearly contact, a repulsive force occurs therebetween due to the Pauli exclusion principle. The attractive and repulsive forces between the atoms are very weak, typically about 10xe2x88x927 to 10xe2x88x9212 N, and thus can be difficult to detect.
In general, the AFM probe is positioned a distance from the sample surface that is within a range in which the cantilever is deflected by the inter-atomic force on the probe tip. Then when the probe is scanned along the sample surface, the distance between the probe and the sample varies depending upon the configuration of the sample surface and, accordingly, the amount of deflection of the cantilever varies. Typically, this movement of the probe tip follows the topography of the sample surface. This variation in deflection of the cantilever is detected, and feedback control is effected by use of a fine movement element, such as a piezoelectric element, to return the amount of deflection of the cantilever to an initial, set point value. Based on the voltage applied to the piezoelectric element by the feedback system in response to the displacement of the probe tip, an image of the surface configuration of the sample can be obtained.
Typically, the cantilever employed in the AFM, or magnetic force microscope (MFM), or other scanning probe microscope (SPM) or profiler is a beam that is fixed at one end and free to bend at the other end. The dimensions (length, width, thickness) and Young""s modulus determine the spring constant which may be selected to ensure that the cantilever exhibits high responsiveness to weak inter-atomic or magnetic forces such that the system is sensitive to cantilever deflection. Notably, various techniques have been employed to detect this cantilever deflection. In particular, various tip sensors are known in the prior art and include those that utilize tunneling currents, optical interferometry, or optical lever, etc. An AFM implemented with a tunneling sensor includes a probe having a sharply pointed tip that is attached to a spring-like cantilever beam to scan the topography of a surface to be investigated. The attractive or repulsive forces occurring between the atoms at the apex of the tip and those of the surface result in small deflections of the cantilever beam, as described above. The deflection may be measured by a tunneling microscope, which includes an electrically conductive tunnel tip that is disposed from the sample surface a particular distance. In operation, a tunneling current is measured as the tunnel tip is scanned over the sample surface. Variations in the tunneling current are then monitored, with the variations being indicative of cantilever beam deflection. Using these beam deflection measurements in conjunction with known characteristics of the cantilever, the tunneling sensor determines the forces between the tip and the surface under investigation. An AFM implemented with a tunneling sensor is described by G. Binnig et al, in Phys. Rev. Lett., vol. 56, No. 9, March 1986, pp. 930-933.
Alternatively, optical methods may be implemented. For example an AFM implemented with a laser interferometer can be used to measure the tip displacement. See, generally, G. McClelland et al., entitled xe2x80x9cAtomic Force Microscopy: General Principles and a New Implementationxe2x80x9d, Rev. Progr. Quart. Non-destr. Eval., Vol. 6, 1987, p. 1307, and Y. Martin et al., entitled xe2x80x9cAtomic Force Microscope-Force Mapping and Profiling on a Sub 100-A scalexe2x80x9d, J. Appl. Phys., vol. 61, no. 10, May 15, 1987, pp. 4723-4729. Laser interferometers utilize optical beam splitters to separate a laser beam into a plurality of beam components which includes the primary beam components of interest, and a photosensor to detect an interfering component of the primary beam components. This interfering component is indicative of cantilever displacement. The advantages of optical detection over tunneling detection include increased reliability and ease of implementation, insensitivity to the roughness of the beam, and a smaller sensitivity to thermal drift.
Another optical deflection method includes using an optical lever in conjunction with a laser beam directed towards the back of the cantilever. Notably, to achieve good sensitivity with the optical lever, the incident angle of light on the reflective surface of the probe with respect to the longitudinal axis of the cantilever should be large, with the best sensitivity theoretically occurring when the angle of incidence is 90xc2x0. The sensitivity falls off with the sine of the angle of incidence, and drops to zero when the angle of incidence is zero. Importantly, for known optical lever systems which reflect the light beam off the back of the cantilever, it is necessary to place the laser directly above the cantilever. Such an arrangement is disclosed in U.S. Pat. No. 5,497,656.
FIG. 1 shows a schematic representation of an AFM 1 having a conventional cantilever beam deflection apparatus 2 for detecting the deflection of a probe assembly 3 as a tip 4 of the probe assembly interacts with a surface 5 of a sample 6. Notably, sample 6 is mounted on a scanner 7 which moves sample 6 to allow AFM 1 to scan surface 5. Probe assembly 3 includes a substrate 8 having a cantilever 9 extending therefrom, while tip 4 extends from the free end of cantilever 9. Deflection apparatus 2 includes a laser 10 for directing a beam of light downwardly towards the top surface of cantilever 9 in a direction generally perpendicular to sample surface 5. During operation, cantilever 9 reflects the laser beam towards a mirror 12 which directs the beam through a collecting lens 13 and towards a position sensing detector 14. Optical beam deflection detection apparatus 2 then measures the position of the deflected light beam which is indicative of the deflection of the cantilever. The deflection of cantilever 9 is a measure of the interaction force between tip 4 and surface 5 of sample 6. Although this straight-forward arrangement is useful in some applications, in many applications (for example, near-field scanning optical microscopy, discussed below), the arrangement shown in FIG. 1 has significant disadvantages.
For example, it is known to combine an AFM with a conventional optical microscope to aid in alignment of the laser beam on the back of the cantilever and to provide a view of the surface features of the sample. Notably, high performance microscope objectives have a short working distance and must be positioned close to the sample surface. High resolution optical imaging is therefore difficult to implement in combination with the optical lever (or an interferometer) because there is not enough room for both the laser source and/or beam and a high-performance microscope objective. A similar problem exists with near-field scanning optical microscopy when using a solid immersion lens and a microscope objective. See, for example, U.S. Pat. No. 5,939,709 to Ghislain et al. and entitled xe2x80x9cScanning Probe Optical Microscope Using a Solid Immersion Lens,xe2x80x9d which is expressly incorporated herein by reference. Some systems have attempted to overcome this limitation by directing a laser through the microscope objective, or by providing multiple reflective surfaces and a photodetector positioned so as to accommodate the microscope objective. However, such systems have significant drawbacks including, in the case of the former system, increased noise with attendant poor image quality, and in the latter system, unreliable detection and alignment problems due to an increased path length of the reflected light. Alternatives to detection using optical beam deflection have been attempted, including using strain gauges and resistive elements disposed on the cantilever, but each has achieved little, if any, practical success due to substantial noise effects that compromise the integrity of the images produced.
In view of the above-noted shortcomings, the state of the art of atomic force microscopy was in need of a system that provides reliable cantilever deflection and accommodates various cantilever designs and related imaging apparatus. A system which could accommodate a high performance microscope objective, while providing reliable force feedback control of the cantilever, would be particularly desirable.
The preferred embodiments of the present invention obviate the above-noted problems by providing an optical deflection detection apparatus that directs light from a source disposed at a position other than superjacent to the cantilever, and by providing an efficient light directing element on the free end of the cantilever to minimize noise effects and maximize deflection sensitivity. The preferred embodiments take advantage of the discovery that deflection of the cantilever can be detected equally well from a position essentially in-line with the end of the cantilever (as well as from other locations) as from a position above the cantilever. The light directing element of the cantilever has a reflecting surface that is disposed at an angle relative to the longitudinal axis of the cantilever beam. The preferred embodiment therefore permits the deflection detection apparatus to be positioned at any of a plurality of locations relative to the cantilever. As a result, the apparatus can be configured to provide clearance above the cantilever to make room for an objective lens of an optical microscope, or to combine the AFM with a near-field scanning optical microscope, or to provide a space for other instrumentation or hardware, or optical or other access to the sample without compromising the noise performance of the AFM.
According to a first aspect of the preferred embodiment, a method of scanning probe microscopy includes using a cantilever having a planar body, generally opposed first and second ends, and a tip disposed generally adjacent the second end and extending towards a surface of a sample. Preferably, the sample is disposed on a support surface. Next, the method includes directing a beam of light onto the second end in a direction substantially parallel to the support surface. In operation, the second end directs the beam towards a detector apparatus at a particular angle. The method monitors a change in the angle of deflection of the beam of light caused by deflection of the cantilever as the cantilever tip traverses the surface of the sample, the change being indicative of a characteristic of the surface.
According to another aspect of the preferred embodiment, the second end includes a flat reflective surface, with the flat reflective surface being generally non-planar with respect to the planar body of the cantilever. In one embodiment, the flat reflective surface comprises a mirror fixed to the second end, while in another embodiment, the flat reflective surface is microfabricated integrally with the cantilever.
According to yet another aspect of the invention, a method of scanning probe microscopy includes using a cantilever having a planar body, generally opposed first and second ends, and a tip disposed generally adjacent to the second end. In this embodiment, the tip preferably extends towards the surface of a sample disposed on a support surface. This method then involves directing a beam of light towards the second end in a direction substantially non-perpendicular to the support surface, whereby the second end directs the beam towards a detector. The method monitors a change in an angle at which the directed beam impinges upon the detector, the change caused by deflection of the cantilever in response to interaction between the tip and the sample surface.
According to a further aspect of the preferred embodiment, a deflection detection apparatus for a scanning probe microscope includes a cantilever having (1) a planar body having top and bottom surfaces, (2) generally opposed first and second ends, and (3) a tip disposed generally adjacent the second end and extending towards a surface of a sample. Again, the sample preferably is disposed on a support surface. The apparatus also includes a light source to direct a beam of light towards the second end in a direction substantially parallel to the support surface, with the second end including a light directing element to direct the beam. Next, a detector is included to sense the directed beam, wherein a change in an angle at which the directed beam contacts the detector is indicative of a deflection of the cantilever.
According to another aspect of the invention, a scanning probe optical microscope for imaging a surface of a sample disposed on a support surface includes a cantilever having a generally planar body and opposed fixed and free ends, the cantilever including a solid immersion lens made of a high index of refraction material. The solid immersion lens has a first surface to receive light and a second surface forming a probe tip. The solid immersion lens optically images the sample surface so as to generate an optical image signal. The microscope also includes a force feedback apparatus having (1) a light source that directs a beam of light towards the free end in a direction generally non-perpendicular to the support surface, the free end including a light directing element to direct the beam, and (2) a detector that receives the directed beam and generates a feedback signal indicative of a change in the angle at which the directed beam impinges upon the detector. Preferably, the optical image signal and the feedback signal are generated simultaneously.
These and other objects, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.